1. Field of the Invention
The invention concerns a method of forming a film by extruding a molten polymer material through a flat monoextrusion die, and the apparatus for carrying out this method.
2. Description of the Related Art
By monoextrusion of film it is meant extrusion of a mono-component, monolayered film as opposed to the coextrusion of bicomponent or multicomponent film. Monocomponent, monolayered film extruded from a flat die, is used in itself, e.g. for manufacture of plastic bags or as wrapping film for a variety of packaging purposes. It is also used, normally in form of a very thin and still molten film, as it comes out of the flat die, for coating or extrusion lamination of other film materials.
The present invention is conceived with a particular view to such coating or extrusion lamination applications, although it also with advantage can be used in itself for bag making, wrapping etc.
To the knowledge of the inventor, all industrially flat extrusion dies—when they are of a considerable width, e.g. 1 m or more from one film edge to the other—are so called “coat hanger dies”. In such wide, flat dies, the polymer flow coming from a reservoir such as an extruder is fed into a chamber from its middle, and flows transversely towards both sides of the chamber. This is integral with the exit slot. The chamber narrows down towards its two sideways ends so that the transverse flow maintains an almost constant velocity.
The gap of the exit slot in the “coat hanger die” is adjustable by means of a row of screws or similar to compensate for the relatively big pressure differences between the middle and each side end of the channel. When such coat hanger dies are used for coating or for extrusion lamination, there are difficulties in achieving the very low film thicknesses, which normally are desirable. This requires high temperatures at which the polymer material tends to degrade or cross-link or both. To avoid or reduce these effects, high throughputs and therefore particular high take off velocities of the film is needed.
In many cases this is not rational, since it makes the machine costs high. Thus, lower throughputs would often be preference if any tendency towards stagnation and similar could be avoided.